Query on renewing Software Maintenance Service

I have a student license of MATLAB with the PDE toolbox add-on. I bought the set last year. I have a question regarding renewal of the software maintenance service.

This year, I want to renew the maintenance service for MATLAB only, and not for the PDE toolbox. Say the price of renewal of the PDE toolbox only is $x this year, and it increases to $y next year. Next year, if I want to renew the PDE toolbox, do I have to pay $y or $?